Saturday, 22 September 2012

Weirdly Wonderful Wongwibinda

I finally found them, photos of some of the strange metamorphic rock at Wongwibinda. I recently moved house and in the process I’ve lost many things but also found some things. Early this year I did a post on what were the broader conditions that lead to the geology of this area between Guyra and Ebor, namely thinning of the continental crust leading to increased heat flow and corresponding thermal metamorphism. I mentioned a rock type called migmatite and since I found my photos of the Wongwibinda migmatite, I thought I should go into a little more detail on this curious metamorphic feature.

Close angular folds in the Girakool Beds, Rockvale
The migmatites are strongly metamorphosed rocks of the Girrakool beds. The Girrakool beds are Carboniferous in age and were deposited in a marine environment. These beds were then accreted onto the edge of the Australian continent as part of the New England Orogen, much deformation occurred during this time. During or following this stage of tectonic forces that affected the New England region the Girrakool beds were subjected to a period of intense metamorphism. This affected one end of the beds more than the other. The western most part of the Girrakool beds in the Rockvale area remained relatively ‘uncooked’ but further to the east the effects of thermal metamorphism became greater creating schists known as the Ramspeck Schist and finally the zone of migmatites. The migmatites are faulted off by the Wongwibinda fault on the eastern side or are intruded by the Abroi Granodiorite which itself has been later metamorphosed into Gneiss.

Migmatite in the Aberfoyle-Wongwibinda area.
Note the ptygmatic folds and dyke on the left
The odd thing about the Wongwibinda migmatites generally is that they are actually three rocks in one: metamorphic sedimentary rocks becoming igneous at the same time. Usually rocks fit into the igneous and sedimentary categories neatly and then metamorphism can affect these rocks. In the case of migmatite the metamorphism is so great that the rock actually begins to melt, that is, it becomes an igneous rock with some of the sedimentary rock remaining unmelted. A characteristic of migmatite is ptygmatic folding, which is intense small scale folding with alternating light and dark bands. The dark bands are called the palaeosome which is the remains of the sedimentary rock and the lighter bands is insitu accumulation of melted rock called the Leucosome,. The leucosome is here comprised mainly of the minerals quartz, feldspar, mica and some garnet. Sometimes the leucosome can ‘break free’ from the ptygmatic folds and create dyke like structures. All of these features are visible in the picture opposite. 

What can be seen at Wongwibinda is essentially the formation of a granite, specifically a S-type (sedimentary derived), frozen in time. Craven et al 2012 demonstrated that this time was very close to the Carboniferous-Permian age boundary, probably just in the Permian, that is around 297 million years ago. There are some fancy geological features in the New England highlands and in my mind this is one of them. If you travel up that way and see some rocks by the side of the road be sure to stop and look closely, there are so many unusual things to find.

*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.

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